From Thomas Moore's Servlet FAQ:

Form-based file upload requires a couple of steps.

The server must supply (and the client must support) encoding type multipart/form-data. Most current browsers do, but it's not a guarantee. Secondly (and this is usually the trickiest part), your servlet has to parse the binary data and do something with it (e.g., write it to a file on the server).

The intrepid programmer is referred to RFC 1867 for cluefulness on how to parse this data. Less brave souls can use either Jason Hunter's implementation of a MultipartRequest (available from http://www.servlets.com), or CParseRFC1867 (available from http://www.servletcentral.com).

Note that the source code is available for both of these examples, but both assume that you will be writing the file to a file on the server. Other uses (e.g. storing the file as a binary object in a database) will require adaptation.

There is a multipart/form parser availailable from Anders Kristensen (http://www-uk.hpl.hp.com/people/ak/java/, [email protected]) at http://www-uk.hpl.hp.com/people/ak/java/#utils. JavaMail also has MIME-parsing routines (see the Purple Servlet References).

Here is an example of HTML code that allows file upload, courtesy of Detlef Pleiss ([email protected]):

<FORM ENCTYPE="multipart/form-data" 
  method=post action="/myservlet"> 
<INPUT TYPE="file" NAME="mptest">
<INPUT TYPE="submit" VALUE="upload">
</FORM>

The input type "file" brings up a button for a file select box on the browser together with a text field that takes the file name once selected. The servlet uses the GET method parameters to decide what to do with the upload while the POST body of the request contains the file data to parse. Tested with IE4, IE5 and Netscape 4.5.